Diffraction In Wireless Communication

• Diffraction In Wireless Communication System
• Diffraction In Wireless Communication Definition
• Diffraction In Wireless Communication
• Reflection Diffraction And Scattering In Wireless Communication Ppt
• Diffraction In Wireless Communication Ppt

Diffraction is a phenomenon where electromagnetic waves (such as light waves) bend around corners to reach places which are otherwise not reachable i.e. not in the line of sight. In technical jargon such regions are also called shadowed regions (the term again drawn from the physics of light). This phenomenon can be explained by Huygen's principle which states that 'as a plane wave propagates in a particular direction each new point along the wavefront is a source of secondary waves'. This can be understood by looking at the following figure. However one peculiarity of this principle is that it is unable to explain why the new point source transmits only in the forward direction.

The unique properties of surface plasmons enable wireless transmission of signals separated by less than one wavelength. Until recently, it was considered impossible to distinguish signals with. Diffraction Loss Using Knife-Edge Model Author: John (YA) John has over 20 years of Research and Development experience in the field of Wireless Communications.

The electromagnetic field in the shadowed region can be calculated by combining vectorially the contributions of all of these secondary sources, which is not an easy task. Secondly, the geometry is usually much more complicated than shown in the above figure. For example consider a telecom tower transmitting electromagnetic waves from a rooftop and a pedestrian using a mobile phone at street level. The EM waves usually reach the receiver at street level after more than one diffraction (not to mention multiple reflections). However, an approximation that works well in most cases is called knife edge diffraction, which assumes a single sharp edge (an edge with a thickness much smaller than the wavelength) separates the transmitter and receiver.

The path loss due to diffraction in the knife edge model is controlled by the Fresnel Diffraction Parameter which measures how deep the receiver is within the shadowed region. A negative value for the parameter shows that the obstruction is below the line of sight and if the value is below -1 there is hardly any loss. A value of 0 (zero) means that the transmitter, receiver and tip of the obstruction are all in line and the Electric Field Strength is reduced by half or the power is reduced to one fourth of the value without the obstruction i.e. a loss of 6dB. As the value of the Fresnel Diffraction Parameter increases on the positive side the path loss rapidly increases reaching a value of 27 dB for a parameter value of 5. Sometimes the exact calculation is not needed and only an approximate calculation, as proposed by Lee in 1985, is sufficient.

Fresnel Diffraction Parameter (v) is defined as:

v=h√(2(d1+d2)/(λ d1 d2))

where

d1 is the distance between the transmitter and the obstruction along the line of sight

d2 is the distance between the receiver and the obstruction along the line of sight

h is the height of the obstruction above the line of sight

and λ is the wavelength

The electrical length of the path difference between a diffracted ray and a LOS ray is equal to φ=(π/2)(v²) and the normalized electric field produced at the receiver, relative to the LOS path is e^-jφ. Performing a summation of all the exponentials above the obstruction (from v to positive infinity) gives us the Fresnel Integral, F(v).

Diffraction In Wireless Communication System

The MATLAB codes used to generate the above plots are given below (approximate method followed by the exact method). Feel free to use them in your simulations and if you have a question drop us a comment.

We have used the following equations in the exact calculation of the Diffraction Loss [1] above. We did not want to scare you with the math so have saved it for the end.

Also please checkout this interesting video explaining the phenomenon of diffraction.

[1] http://www.waves.utoronto.ca

Author: John (YA)

John has over 20 years of Research and Development experience in the field of Wireless Communications. He has worked for a number of companies around the world including Qualcomm Inc. USA.

Radio Propagation Tutorial Includes:
Radio propagation basicsRadio signal path lossFree space propagation & path lossLink budgetRadio wave reflectionRadio wave refractionRadio wave diffractionMultipath propagationMultipath fadingRayleigh fadingThe atmosphere & radio propagation

Radio path loss is key factor in the design of any radio communications system or wireless communication system.

It is a fact that any radio signal will suffer attenuation when it travels from the transmitter to the receiver. A variety of different phenomena give rise to this radio path loss.

Diffraction In Wireless Communication Definition

Understanding what causes radio path loss enables any system to be designed to perform to its best despite the various issues affecting it.

How does radio path loss affect systems

The radio signal path loss will determine many elements of the radio communications system or wireless communication system in particular the transmitter power, and the antennas, especially their gain, height and general location. This is true for whatever frequency is used.

Fresnel Diffraction Parameter (v) is defined as:

v=h√(2(d1+d2)/(λ d1 d2))

where

d1 is the distance between the transmitter and the obstruction along the line of sight

d2 is the distance between the receiver and the obstruction along the line of sight

h is the height of the obstruction above the line of sight

and λ is the wavelength

The electrical length of the path difference between a diffracted ray and a LOS ray is equal to φ=(π/2)(v²) and the normalized electric field produced at the receiver, relative to the LOS path is e^-jφ. Performing a summation of all the exponentials above the obstruction (from v to positive infinity) gives us the Fresnel Integral, F(v).

Diffraction In Wireless Communication System

The MATLAB codes used to generate the above plots are given below (approximate method followed by the exact method). Feel free to use them in your simulations and if you have a question drop us a comment.

We have used the following equations in the exact calculation of the Diffraction Loss [1] above. We did not want to scare you with the math so have saved it for the end.

Also please checkout this interesting video explaining the phenomenon of diffraction.

[1] http://www.waves.utoronto.ca

Author: John (YA)

John has over 20 years of Research and Development experience in the field of Wireless Communications. He has worked for a number of companies around the world including Qualcomm Inc. USA.

Radio Propagation Tutorial Includes:
Radio propagation basicsRadio signal path lossFree space propagation & path lossLink budgetRadio wave reflectionRadio wave refractionRadio wave diffractionMultipath propagationMultipath fadingRayleigh fadingThe atmosphere & radio propagation

Radio path loss is key factor in the design of any radio communications system or wireless communication system.

It is a fact that any radio signal will suffer attenuation when it travels from the transmitter to the receiver. A variety of different phenomena give rise to this radio path loss.

Diffraction In Wireless Communication Definition

Understanding what causes radio path loss enables any system to be designed to perform to its best despite the various issues affecting it.

How does radio path loss affect systems

The radio signal path loss will determine many elements of the radio communications system or wireless communication system in particular the transmitter power, and the antennas, especially their gain, height and general location. This is true for whatever frequency is used.

To be able to plan the system, it is necessary to understand the reasons for radio path loss, and to be able to determine the levels of the signal loss for a given radio path.

The radio path loss can often be determined mathematically and these calculations are often undertaken when preparing coverage or system design activities. These depend on a knowledge of the signal propagation properties.

Accordingly, radio path loss calculations are used in many radio and wireless survey tools for determining signal strength at various locations. These wireless survey tools are being increasingly used to help determine what radio signal strengths will be, before installing the equipment. For cellular operators radio coverage surveys are important because the investment in a macrocell base station is high. Also, wireless survey tools provide a very valuable service for applications such as installing wireless LAN systems in large offices and other centres because they enable problems to be solved before installation, enabling costs to be considerably reduced. Accordingly there is an increasing importance being placed onto wireless survey tools and software.

Diffraction In Wireless Communication

Radio path loss basics

The signal path loss is essentially the reduction in power density of an electromagnetic wave or signal as it propagates through the environment in which it is travelling. This affects all radio communication, broadcast and wireless communiction systems

There are many reasons for the radio path loss that may occur:

• Free space loss: The free space loss occurs as the signal travels through space without any other effects attenuating the signal it will still diminish as it spreads out. This can be thought of as the radio communications signal spreading out as an ever increasing sphere. As the signal has to cover a wider area, conservation of energy tells us that the energy in any given area will reduce as the area covered becomes larger.
• Diffraction: radio signal path loss due diffraction occurs when an object appears in the path. The signal can diffract around the object, but losses occur. The loss is higher the more rounded the object. Radio signals tend to diffract better around sharp edges, i.e. edges that are sharp with respect to the wavelength.
• Multipath: In a real terrestrial environment, signals will be reflected and they will reach the receiver via a number of different paths. These signals may add or subtract from each other depending upon the relative phases of the signals. If the receiver is moved the scenario will change and the overall received signal will be found vary with position. Mobile receivers (e.g. cellular telecommunications phones) will be subject to this effect which is known as Rayleigh fading.
• Absorption losses: Absorption losses occur if the radio signal passes into a medium which is not totally transparent to radio signals. There are many reasons for this which include:
  
  • Buildings, walls, etc: When radio signals pass through dense materials such was walls, buildings or even furniture within a building, they suffer attenuation. It is particularly applicable to cellular communications – in buildings, houses, etc signals are considerably reduced. The radio signal attenuation is more pronounced for the higher frequency mobile bands., e.g. 2.2 GHz as opposed to 800 / 900 MHz.
  • Atmospheric moisture: At high microwave frequencies radio path loss increases as a result of precipitation or even moisture in the air. The radio signal path loss may vary according to the weather conditions. However this typically only has a noticeable effect further into the microwave region.
  • Vegetation: In dense forest it is found that signals even at lower frequencies are considerably reduced. This illustrates that vegetation can introduce considerable levels of radio path loss. Trees and foliage can attenuate radio signals, particularly when wet.
• Terrain: The terrain over which signals travel will have a significant effect on the signal. Obviously hills which obstruct the path will considerably attenuate the signal, often making reception impossible. Additionally at low frequencies the composition of the earth will have a marked effect. For example on the Long Wave band, it is found that signals travel best over more conductive terrain, e.g. sea paths or over areas that are marshy or damp. Dry sandy terrain gives higher levels of attenuation.
• Atmosphere: The atmosphere can affect radio signal paths.
  • Ionosphere: At lower frequencies, especially below 30 - 50MHz, the ionosphere has a significant effect, reflecting (or more correctly refracting) them back to Earth. However when passing through some regions, especially the D region and to a lesser extent the E region, signals can suffer attenuation rather than reflection / refraction. This can introduce a significant radio path loss.
  • Troposphere: At frequencies above 50 MHz and more the troposphere has a major effect, refracting the signals back to earth as a result of changing refractive index. For UHF broadcast this can extend coverage to approximately a third beyond the horizon. The refraction can sometimes mean that signal that would normally reach a certain area may be refracted away from it.

These reasons represent some of the major elements causing signal path loss for any radio system.

Predicting radio path loss

One of the key reasons for understanding the various elements affecting radio signal path loss is to be able to predict the loss for a given path, or to predict the coverage that may be achieved for a particular base station, broadcast station, etc.

Although prediction or assessment can be fairly accurate for the free space scenarios, for real life terrestrial applications it is not easy as there are many factors to take into consideration, and it is not always possible to gain accurate assessments of the effects they will have.

Reflection Diffraction And Scattering In Wireless Communication Ppt

Despite this there are wireless survey tools and radio coverage prediction software programmes that are available to predict radio path loss and estimate coverage. A variety of methods are used to undertake this.

Free space path loss varies in strength as an inverse square law, i.e. 1/(range)², or 20 dB per decade increase in range. This calculation is very simple to implement, but real life terrestrial calculations of signal path loss are far more involved. To show how a real life situation can alter the calculations, often mobile phone operators may modify the inverse square law to 1/(range)ⁿ where n may vary between 3.5 to 5 as a result of the buildings and other obstructions between the mobile phone and the base station.

Most path loss predictions are made using techniques outlined below:

• Statistical methods: Statistical methods of predicting signal path loss rely on measured and averaged losses for typical types of radio links. These figures are entered into the prediction model which is able to calculate the figures based around the data. A variety of models can be used dependent upon the application. This type of approach is normally used for planning cellular networks, estimating the coverage of PMR (Private Mobile Radio) links and for broadcast coverage planning.
• Deterministic approach: This approach to radio signal path loss and coverage prediction utilises the basic physical laws as the basis for the calculations. These methods need to take into consideration all the elements within a given area and although they tend to give more accurate results, they require much additional data and computational power. In view of their complexity, they tend to be used for short range links where the amount of required data falls within acceptable limits.

These wireless survey tools and radio coverage software packages are growing in their capabilities. However it is still necessary to have a good understanding of radio propagation so that the correct figures can be entered and the results interpreted satisfactorily.

For any given radio transmission, the radio path loss is likely to be caused by a number of different factors. This often makes accurate radio path loss calculations difficult. However even if they are not as accurate as might be always liked, the radio path loss calculations enable equipment to be designed to meet the requirements.

Diffraction In Wireless Communication Ppt

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